Oscillators are electrical devices that generate an oscillating or repetitive signal (oscillations). Oscillations comprise a voltage which varies in magnitude and sign over time. Oscillations can be a sinusoidal wave, such as in an analog signal, or a square wave, such as in a digital electronic signal. Oscillations generated by an oscillator, especially electronic signals, have a number of applications such as, for example, a precise reference clock source in a voltage-controlled oscillator for frequency tuning, a reference clock source in a phase-locked loop (PLL) for locking onto another signal, or a frequency synthesizer to generate many other frequency references required in specific applications including microprocessors, wireline (tethered) or wireless communication systems, and application-specific integrated circuits (ASICs).
Oscillators comprise a resonator and an oscillator core. The resonator creates the oscillations and the oscillator core provides power to the resonator to initiate and sustain oscillations. A resonator can be, for example, an inductor-capacitor (LC) resonator or an electro-mechanical resonator. LC resonators comprise an inductor and a fixed capacitor in a series or parallel configuration. A variable capacitor can also be added to an LC resonator to tune the frequency of oscillations produced by an oscillator comprising an LC resonator. Compared to an electro-mechanical resonator, an LC resonator is typically better suited for oscillators where the frequency needs to be tunable.
The use of an electro-mechanical resonator, such as a piezoelectric resonator, in place of an LC resonator can improve the quality (spectral purity) of the oscillations in an oscillator. The quality factor (as referred to as Q factor, and Q) of a resonator determines how damped its oscillator is—the higher the quality factor, the lower the rate of energy loss relative to the stored energy of the resonator. LC resonators in an integrated circuit (IC), for example, have a quality factor between 5 and 25. The quality factor of an electro-mechanical resonator can be 10 to 100 times higher than that of an integrated LC resonator.
When an electro-mechanical resonator is used with a differential oscillator, that has a common-source cross-coupled transistor oscillator core, to produce balanced oscillations, however, issues are introduced with respect to the oscillator latching to a static, non-oscillatory, direct-current (DC) stable state. Unlike an LC resonator, an electro-mechanical resonator has a very high impedance at low frequency and acts like an open circuit at DC. Although not an issue for single-ended oscillators, the high impedance at DC causes the cross-coupled transistors in a differential oscillator to become a latch with a very high DC gain so as to prevent the oscillations from starting in the oscillator. Accordingly, electro-mechanical resonators are commonly used in three-point (also known as single-ended) oscillator topologies, such as Colpitts, Pierce, and Hartley oscillators, which do not suffer from the latching problem.
FIGS. 1A, 1B and 1C show three-point electro-mechanical oscillators. Three point oscillators, however, only provide a single-ended output signal, not a differential output signal. The differential output signals, as produced by a cross-coupled oscillator with an LC resonator, have a better common-mode noise rejection and an increased oscillation swing across the resonator as compared to the single-ended output signal. The increased oscillation swing improves signal-to-noise ratio (SNR) and hence the oscillator's phase noise.
One known approach to address the latching issue is to place a degeneration capacitor in series with source terminals of the cross-coupled differential pair NMOS (or PMOS) transistors. This breaks the loop formed by the differential pair transistors and the resonator at DC, while closing the loop as desired at high frequencies. Source degeneration capacitors, however, cannot be used with oscillators comprising complementary cross-coupled inverters where each inverter comprises an NMOS and a PMOS transistor forming a complementary metal-oxide-semiconductor (CMOS) inverter gain stage. There are potential advantages to using complementary cross-coupled inverters in an oscillator such as, for example, boosting transconductance gain (gm) and improving the oscillation swing and phase noise. Adding capacitors to the source with cross-coupled complementary oscillators comprising a pair of NMOS and PMOS transistors would decrease the signal swing and phase noise performance of oscillations in the oscillator. Furthermore, placing a capacitor in parallel with inverters and connected to the source of the transistors could result in unwanted parasitic relaxation oscillations. Whether relaxation oscillations occur depends on the resistance and capacitance values in the DC blocking path of the oscillator. Stability analysis can be performed to determine the largest capacitor possible to avoid relaxation oscillations, but at the expense of lower signal swing and worse phase noise performance, as well as increased design complexity. Accordingly, it would be desirable to have a cross-coupled complementary oscillator comprising an electro-mechanical resonator that does not latch to DC or experience relaxation oscillations.
Some oscillator applications, such as in telecommunications or instrumentation, require oscillations with a very precise and accurate frequency to the order of tens of parts per million (ppm) or smaller. Resonators that are built into oscillators, however, can have their resonance frequency vary in the order of hundreds of ppm to thousands of ppm for various reasons including, without limitation, fluctuations in temperature, manufacturing variations, and degradation of electronic characteristics over time, also known as aging. A variable capacitor may be placed in parallel with the resonator and the oscillator core to tune the oscillations to the desired frequency using a control voltage applied to the variable capacitor. The capacitance along with the parasitic trace or package inductance from attaching the capacitor to the circuit or other inductances can resonate together, however, and cause the oscillator to oscillate at undesired parasitic frequencies (also referred to as parasitic mode oscillations or parasitic package-mode oscillations) rather than at the resonator frequency. This is because the parasitic inductance and variable capacitance structures have a lower quality (Q) factor than the resonator allowing the parasitic oscillations to build more quickly in the oscillator than the desired resonator frequency oscillations.
Another issue is that the tuning range of an oscillator comprising the electro-mechanical resonator is much narrower than the tuning range of an oscillator comprising an LC resonator. The main reason for this is a superior frequency selectivity of an electro-mechanical resonator, which comes from its large Q factor, and in turn translates to superior phase noise performance, but inherently restricts its frequency tunability. As explained later in relation to FIG. 15, the tuning range of an electro-mechanical oscillator is limited to a narrow range between series resonance frequency 1508 (fs) and parallel resonance (or anti resonance) frequency 1510 (fp) of its electro-mechanical resonator.
Yet another issue is that thermal noise from the cross-coupled inverters gets translated to phase noise in the oscillations by a mechanism known as “amplitude modulation to phase modulation” (or AM-to-PM) conversion due to the modulation of the capacitance contributed by the varactors. The wider the oscillator's tunability, the more susceptible the oscillator is to AM-to-PM conversion and phase noise degradation. AM-to-PM conversion is a well-known phenomenon by which amplitude noise on oscillator nodes is converted to phase noise, mostly but not limited to because of voltage variable capacitors and supply voltage dependent stray capacitors connected to the oscillation tank. In a varactor, capacitance value is a function of the voltage across the varactor. Self-induced thermal noise and noise generated by devices connected to varactors result in amplitude modulation of the voltage across varactors leading to modulation of capacitance offered by varactors. Since the oscillation frequency and therefore its integral, namely “phase”, are a function of the capacitance in the oscillation tank, which in turn is modulated by the amplitude noise, the AM (amplitude modulation) due to thermal noise is in this way converted to PM (phase modulation) and thus phase noise.
Parasitic mode oscillations tend to occur at frequencies higher than the resonator frequency. Accordingly, it would be desirable to ensure a tunable oscillator comprising an electro-mechanical resonator does not suffer from parasitic mode oscillations that can corrupt the oscillator's spectral purity.